Re: Specifying names for tap interfaces

[Roger Pau Monne <roger.pau%citrix.com@localhost>]

Jean-Yves Migeon wrote:
> On 21.06.2012 17:10, Manuel Bouyer wrote:
> > On Thu, Jun 21, 2012 at 12:16:17PM +0100, Roger Pau Monne wrote:
> > > As I said on a previous email, how can you guarantee that the name
> > > you chose and pass to Qemu will not be taken by someone else while
> > > you are calling Qemu?
> > >
> > > You might even be launching several guests at the same time, which
> > > will lead you to a race.
> > This is a problem with name, but not if you use the fd-type setup:
> > - xl opens /dev/tap, which creates the tap device (so xl knows the name) and
> >    a file descriptor (which I'll call n)
> > - fork and exec qemu with: -net tap,vlan=0,fd=n
> > - the xl parent process closes n.
> >
> > in this setup, qemu reuses the already-existing tape device and doesn't
> > create a new one, so no race condition possible here.
> > This should fix the problem.
>
> I like this idea, I did not know we could pass down a fdes to Qemu.
>
> I would recommend this if it works. That gives us some time to think
> about interfaces/devices xaliases instead of rushing it for
> Qemu-upstream support.

Yes, I will try to implement this approach in xl, hopefully I won't have 
to change much of the code, and just add an OS dependant helper to 
choose the network type.